Ontogenetic variation and sexual dimorphism in the East African root rat, Tachyoryctes splendens (Rüppell 1835) from Tanzania, East Africa were examined using craniometric and body mass data. Ontogenetic variation was based on tooth eruption and wear in five relative age classes and revealed significant craniometric variation between them. Sexual dimorphism was evident in all three populations examined and revealed that males are larger in body size than females. The results of craniometric analysis of ontogenetic variation and sexual dimorphism in T. splendens are also reflected in the analysis of body mass data. These results suggest that male T. splendens invest energy into growth to facilitate male-male competition and sexual selection during the breeding season. Females however, show conservative growth with most energy being utilized for the excavation of complex burrow systems to increase foraging efficiency and allow for maternal care. Individuals of relative age classes 4 and 5 are reproductively mature and are thus capable of procreation.